In this photo copied from the 2010 Sleepy Hollow High School yearbook, high school student Andrea Rubello is shown. Police said Rubello, a junior at Hofstra University in Hempstead, N.Y., was shot and killed Friday, May 17, 2013, during a break-in near the college campus. (AP Photo/Sleepy Hollow High School) (The Associated Press)

This undated photo provided by the Nassau County Police Department shows Dalton Smith of Hempstead, N.Y. On Saturday, May 18, 2013, police identified Smith the alleged home invader involved in the fatal slaying of a New York college student early Friday morning. Police say that Smith, who was currently on parole for robbery in the first degree, was the person attempting to rob the off-campus home where Andrea Rebello was shot and killed. (AP Photo/Nassau County Police Department) (The Associated Press)

The Tarrytown, N.Y., home of the family of Hofstra University student Andrea Rubello is seen on Friday, May 17, 2013. Police say Rubello was shot and killed Friday, May 17, 2013, during a break-in near the college campus in Uniondale, N.Y. (AP Photo/Jim Fitzgerald) (The Associated Press)

Hofstra University students gather near the house where another student and an armed intruder were killed during an overnight house break-in next to the campus, Friday, May 17, 2013, in Uniondale, N.Y. (AP Photo/ Louis Lanzano) (The Associated Press)

Officers continue working the scene at the house, left, where a Hofstra University student and an armed intruder were killed during an overnight break-in next to the campus, Friday, May 17, 2013, in Uniondale, N.Y. (AP Photo/ Louis Lanzano) (The Associated Press)

NEW YORK – The police officer who accidentally killed a Long Island college student along with an armed intruder faced the most harrowing decision of a law enforcement career: choosing the split-second moment when the risk is so high that you must act to save a life, says an expert in the field.

"The big question is, how do you know, when someone's pointing a gun at you, whether you should keep talking to them, or shoot?" said Michele Galietta, a professor of psychology at the John Jay College of Criminal Justice who helps train police officers. "That's what makes the job of an officer amazingly difficult."

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She spoke Sunday as Hofstra University students honored 21-year-old Andrea Rebello by wearing white ribbons at their graduation ceremony.

Rebello was killed two days earlier when a masked man walked through the unlocked door of her off-campus home. A police officer aiming at the would-be robber opened fire, hitting the Hofstra junior as well as the ex-convict holding her in a headlock.

On Saturday evening, flags on the Mineola campus were at half-staff and students held a silent outdoor vigil in front of a photo of the young woman. Surrounded by candles and flowers, they sang "Ave Maria."

Rebello's funeral is scheduled for Wednesday in Sleepy Hollow, in Westchester County, north of New York City.

Her life ended in split seconds that forced the veteran police officer to make a fatal decision, but the questions surrounding the student's death are just beginning, along with an internal investigation by the Nassau County Police Department.

The bare facts are simple. Rebello and the intruder, Dalton Smith, died early Friday when the officer fired eight shots, hitting him seven times, with the eighth bullet striking Rebello once in the head, according to county homicide squad Lt. John Azzata.

With a gun pointed at her, Smith "kept saying, 'I'm going to kill her,' and then he pointed the gun at the police officer," according to Azzata.

The would-be robber then made a motion indicating he was about to fire, according to authorities.

The officer acted quickly, saying later that he believed his and Rebello's life were in danger, according to authorities.

No doubt, he was acting to try to save lives — his own and that of the young woman, Galietta said.

But the fallout was tragic.

"What we're asking the cop to anticipate is, 'What is going on in the suspect's mind at the moment?'" she says. "We're always trying to de-escalate, to contain a situation, but the issue of safety comes in first, and that's the evaluation the officer has to make."

In collaboration with the New York City Police Department, Galietta is part of a John Jay program that prepares young officers to react to life-threatening situations. Actors are used to replicate scenarios reflecting reality.

Police tactical manuals are meant to assist officers in making the best decision possible, but in the end, "they're not 100 percent foolproof," Galietta said. "In a situation like that, you can follow procedure, and it doesn't mean it comes out perfectly."

The officer who fired the shots is an eight-year NYPD veteran and has been with Nassau County police for 12 years.